Power steering system having a mechanical safety breaker

Abstract

A mechanical safety breaker for disengaging a mechanical connection between two mechanical members at a time when a force transmitted through the connection of the two members exceeds a predetermined value is provided by employing an overload breaker element made of a ceramic. A ceramic breaker element provides a long time stabilized fracture performance particularly when the load applied thereto is of a repetitive nature.

We claim: 
     
       1. A power steering system for assisting steering operation of a manual steering system having a rack and pinion steering means, a steering shaft connected with the rack-and-pinion means for steering operation thereof, and a steering wheel connected with the steering shaft for driving the steering shaft in either of opposite steering directions by a driver, said power steering system comprising: a motor for generating a force for assisting said steering operation of said manual steering system, and   means for torque transmittingly connecting said motor with said manual steering system, wherein said torque transmitting connection means includes a mechanical safety breaker element for breaking the torque transmitting connection of said motor with the manual steering system when the torque transmitted exceeds a predetermined limit value, said breaker element being made substantially of a ceramic material, said ceramic material substantially maintaining its fracture strength over a relatively long period of use under repeated applications of load.   
     
     
       2. A power steering system according to claim 1, wherein said breaker element is a plate element adapted to transmit said force via a sheering along a cross sectional region at a middle portion thereof in a plane extension thereof. 
     
     
       3. A power steering system according to claim 2, wherein said cross sectional region of said plate element is defined by a pair of notches formed in opposite surfaces of said plate element to be substantially aligned to one another across the thickness thereof. 
     
     
       4. A power steering system according to claim 3, wherein said plate element has a sectoral configuration with said notches being formed in parallel with a straight edge thereof to traverse a middle portion of said sectoral configuration, the depth of said notches being changed along a length thereof so as to be smallest at a central portion of the length and to be largest at opposite ends of the length. 
     
     
       5. A power steering system according to claim 4, wherein the depth of said notches is changed arcuately so as to define arcuately convex opposite edges of said cross sectional region. 
     
     
       6. A power steering system according to claim 4, wherein the depth of said notches is changed straightly so as to define straightly convex opposite edges of said cross sectional region. 
     
     
       7. A power steering system according to claim 3, wherein said plate element has a sectoral configuration with said notches being formed in parallel with a straight edge thereof to traverse a middle portion of said sectoral configuration, the depth of said notches being substantially constant along a length thereof. 
     
     
       8. A power steering system according to claim 3, wherein said torque transmitting means includes first and second mechanical members wherein said first mechanical member has a cylindrical outside surface and a substantially radial first key groove opening to said cylindrical outside surface thereof, while said second mechanical member has a cylindrical inside surface to slidably engage with said cylindrical outside surface of said first mechanical member and a substantially radial second key groove opening to said cylindrical inside surface thereof and adapted to align with said first key groove, said plate element being mounted half by half in said first and second key grooves at opposite half portions thereof so as to be subjected to a sheering in the direction of thickness thereof along said cross sectional region by a relative rotational movement of said first and second mechanical members, said pair of notches being arranged such that central planes of said pair of notches join tangentially to a phantom cylindrical curve centred at a central axis of said cylindrical outside surface of said first mechanical member at bottoms of said notches so as thereby to induce a sheering fracture of said plate element along the phantom cylindrical curve. 
     
     
       9. A power steering system according to claim 2, wherein said cross sectional region of said plate element is defined by at least a pair of Knoop indents accompanied by corresponding sectoral cracks formed in opposite surfaces of said plate element to be substantially aligned to one another across the thickness thereof. 
     
     
       10. A power steering system according to claim 9, wherein said cross sectional region of said plate element is defined by at least three pairs of Knoop indents accompanied by corresponding sectoral cracks formed in opposite surfaces of said plate element, each said pair of Knoop indents and the corresponding sectoral cracks being substantially aligned to one another across the thickness thereof, while said at least three pairs of Knoop indents and the corresponding sectoral cracks being consistent to define said cross sectional region. 
     
     
       11. A power steering system according to claim 9, wherein said ceramic breaker element is coated with a layer of a resin. 
     
     
       12. A power steering system according to claim 2, wherein said plate element is an assembly of at least first, second and third plate members, at least said first plate member being made of a ceramic, said second and third plate members being bonded to a surface of said first plate member so as to define a slit therebetween to induce a fracture of said first plate member along a portion thereof aligned with said slit. 
     
     
       13. A power steering system according to claim 2, wherein said cross sectional region of said plate element is defined by a pair of linear scratches formed by a Knoop head to be accompanied by corresponding linear cracks formed in opposite surfaces of said plate element to be substantially aligned to one another across the thickness thereof. 
     
     
       14. A power steering system according to claim 1, wherein said torque transmitting means includes first and second mechanical members wherein said first mechanical member has a cylindrical outside surface and a substantially radial first key groove opening to said cylindrical outside surface thereof, while said second mechanical member has a cylindrical inside surface to slidably engage with said cylindrical outside surface of said first mechanical member and a substantially radial second key groove opening to said cylindrical inside surface thereof and adapted to align with said first key groove, said breaker element being a key mounted half by half in said first and second key grooves at opposite half portions thereof so as to transmit a torque between said first and second mechanical members around an axis corresponding to a central axis of said cylindrical outside surface of said first mechanical member. 
     
     
       15. A power steering system according to claim 14, wherein said first and second key grooves are each formed to define an edge at an opening end thereof which functions as a sheering edge against said key across a middle cross sectional region thereof. 
     
     
       16. A power steering system according to claim 14, wherein at least one of said first and second key grooves is formed with a port space for widening an opening end thereof so as to provide a space in which said key is substantially subjected to a bending stress by a torque transmitted therethrough. 
     
     
       17. A power steering system according to claim 14, wherein said first and second key grooves and said key received therein are provided in duplicate around said central axis such that when a first set of said key grooves and key operates to substantially transmit a torque between said first and second mechanical members, a second set of said key grooves and key idles, and when said key of said first set of said key grooves and key has been fractured, said second set of said key grooves and key operates to substantially transmit a torque between said first and second mechanical members. 
     
     
       18. A power steering system according to claim 14, wherein said key has a stepped thickness so as to present a relatively thicker half portion and a relatively thinner half portion such that when said thicker half portion operates to substantially transmit a torque between said first and second mechanical members, said thinner half portion idles, and when said thicker half portion has been fractured, said thinner half portion operates to substantially transmit a torque between said first and second mechanical members. 
     
     
       19. A power steering system according to claim 1, wherein said torque transmitting means includes first and second mechanical members wherein said first mechanical member has an annular surface adapted to turn about a rotation axis and a first key groove opening to said annular surface thereof, while said second mechanical member has an annular surface opposing said annular surface of said first mechanical member with a substantial space left therebetween and a second key groove opening to said annular surface thereof and adapted to align with said first key groove, said breaker element being a key mounted half by half in said first and second key grooves at opposite half portions thereof with a middle portion thereof traversing said space so as to transmit a torque between said first and second mechanical members around said axis of rotation. 
     
     
       20. A power steering system according to claim 19, wherein said first and second key grooves and said key received therein are provided in duplicate around said central axis such that when a first set of said key grooves and key operates to substantially transmit a torque between said first and second mechanical members, a second set of said key grooves and key idles, and when said key of said first set of said key grooves and key has been fractured, said second set of said key grooves and key operates to substantially transmit a torque between said first and second mechanical members. 
     
     
       21. A power steering system according to claim 1, wherein said torque transmitting means includes first and second mechanical members wherein said first and second mechanical members are each a rotary member adapted to rotate about a common axis of rotation as axially spaced from one another along said axis of rotation, and said breaker element is a member to connect axially opposing end portions of said first and second mechanical members with one another so as to transmit a torque therebetween by bearing a twisting load applied thereto. 
     
     
       22. A power steering system according to claim 1, wherein said torque transmitting means includes first and second mechanical members wherein said first mechanical member is a member to move in unison with a manual steering system of a vehicle, and said second mechanical member is a member to move in unison with an assisting steering power source of the vehicle.